Those notions which are to be collected by reason . . . will seldom stand forward in the mind, but lie treasured in the remoter repositories of memory, to be found only when they are sought.

— Samuel Johnson, The Rambler, 7 April 1759

Sir John Ambrose Fleming (1849-1945) was the inventor of the thermionic valve, devices that formed the glowing (literally!) and pulsing heart of most electronic circuits until the invention of the transistor in the 1960s and the dawn of the Age of Semiconductors.

His part in most GCSE and A-level courses is small in extent but of significant and perhaps under acknowledged importance: he is the original framer of Fleming’s Left Hand Rule and Fleming’s Right Hand Rule. These respectively predict the direction of the force produced on a current-carrying conductor in a magnetic field (left hand) and the direction of induced current flow when a conductor cuts magnetic field lines (right hand). In short, they summarise the physics of everything from the humble electric motor to the Large Hadron Collider via the rail gun; not to mention the giant spinning generators that produce the humming electrical essence that powers our civilisation.

To use the rules, hold your thumb and first two fingers at right angles to each other. I tell my students that the left hand rule and right hand rule are physicists’ gang sign — it’s not too great a stretch of the imagination, at that. If you have ever invigilated a Physics exam, you can tell the point when the students have reached the Fleming’s Left/Right Hand Rule question . . . just look at their hands!

But I digress. I began this post because I was taught the following mnemonic for FLHR:

And to be honest, I have passed it on without thinking too hard about it. However, a student recently introduced me to the F.B.I. Mnemonic. Start with your thumb and say “F for force”, first finger and say “B for B-field” and then second finger and say “I for current”.

6 responses to “The F.B.I. and Gang Signs for Physicists”

Neat! Especially since we spend half our time in mechanics saying that force doesn’t necessarily equal motion. All that’s needed now is an update for “MG is good” for which hand is which, given that the kids today don’t know about sports cars driven by rakish gentleman spies.

One of the odd snippets I explicitly remember from my GCSE teacher (a proper old-school eccentric, at least as a classroom persona… I thought + think he was great). He had a proper, octagonal MG car badge. M is on the left, so it’s left hand for motor. G is on the right, so right hand for generator. Those were the days…